1. Field of the Invention
The present invention relates to casting machines of the type generally referred to as die casting machines.
2. Description of the Prior Art
Die casting machines and processes are well known in the prior art. Generally, a die casting machine consists of a fixed die half and a movable die half which define a die cavity therebetween in the shape of the article to be cast. The die halves also define a gate or runner which is a narrow passageway opening into the die cavity. A vertically or horizontally oriented shot sleeve, having an injection chamber therein, is connected to the fixed die half in such a manner that the outlet end of the injection chamber communicates with the die cavity via the gate or runner. Positioned for reciprocating motion within the injection chamber of the shot sleeve is a plunger. A charge of casting material, such as molten metal, is introduced into the shot sleeve through a suitable opening ahead of the plunger and the plunger is then advanced in the injection chamber to force the charge through the gate or runner and into the die cavity where solidification occurs. The movable die half is thereafter withdrawn so that the solidified casting can be removed.
Prior art die casting machines generally suffer from several disadvantages. Galling and binding between the plunger and shot sleeve is a major problem attributable to the charge, such as molten metal, becoming entrapped in the clearance between the plunger and shot sleeve and solidifying there into abrasive particles or flash. Rapid wear of the plunger and shot sleeve occurs and leads to reduced machine efficiency and component life. As exemplified in U.S. Pat. Nos. 2,932,865 and 3,091,306 prior art workers have attempted to overcome this problem by providing a deformable cup-like extension or sealing member at the plunger face to prevent entrapment of the charge in the clearance. Other problems arise as a result of introducing the molten charge directly into the shot sleeve chamber prior to injection in the die cavity. When molten charge contacts the shot sleeve, the charge may be rapidly chilled, and possibly prematurely solidified. Chilling of a molten metal charge adversely affects its fluidity and ability to form a sound casting. In addition, contact between the shot sleeve and molten charge tends to warp and crack the shot sleeve as a result of temperature differentials set up in the sleeve. Warping and cracking of the shot sleeve are an especially severe problem when metals or alloys having high melting temperatures, such as steels, are being cast.